In my current situation, I like the convenience of the 10-22; I swap lens a little less often. More importantly, I already own one. But, when I do go full frame, the 15mm fisheye would be worth considering, given its size and weight. If I need a zoom, I could always borrow my buddy's 16-35/2.8L II.

I like having both. I have a 17-40L and a 15mm fisheye on my 5D. On my crop camera I have the 10-17 fisheye which I use for both fisheye shots and for rectilinear shots. I consider it good enough for both.